Problems connected with the use of woven cloth as a backing for coated abrasive articles, and for belts in particular, are the elongation characteristic inherent in woven cloth, due to the repeated curvature in the yarns, inherently produced by the interlaced nature of the material, and a weakening of the material in certain circumstances due to the inherent presence of "knuckles" at the crossover points in the yarn. Knuckles are the small bumps on the surface of woven cloth caused by yarns curving to cross over other yarns. The presence of such knuckles is believed to be responsible for the catastrophic failure of coated abrasive article, particularly belts, in certain severe grinding operations.
Stitch-bonded fabrics in general have been known for at least the last twenty years. However, until the invention described in the above referenced copending application, it was not appreciated that such fabrics could confer special advantages when used as the reinforcing substrate for coated abrasive backings. Thus no fabrics explicitly suitable for such purposes were known to the applicant from prior art.
The desirable properties of woven textiles as a backing material for coated abrasives are retained, and many of the undesirable properties are avoided by the use of arrays of substantially coplanar and coparallel textile yarns. Ideal properties for coated abrasives would be expected for backings in which the arrays of yarns are exactly coplanar.
In order to produce stitch bonded fabric in large volume at low cost, it is necessary to use one of the special machines designed for such purposes. A wide variety of machines are available commercially, including those supplied under the trade name Malimo (short for MALIMO Type Malimo) by Unitechna Aushandelgesellschaft mbH of Karl Marx Stadt, GDR, those with the trade name Weft/Loc made by Liba Maschinenfabrik GmbH, D-8674 Naila, FRG, and Raschel knitting machines. (A list of suppliers of Raschel machines is given on pages 31-38 of Volume 43, No. 35 of Knitting Times, the official publication of the National Knitted Outerwear Assoc., 51 Madison Avenue, New York, N.Y., 10010.)
These commercially available machines are normally limited to a maximum number of about one warp yarn per millimeter (mm) of fabric width. This limitation is believed to be necessary to accommodate a sufficient number of stitch or loop forming devices in the machine to form bonds across the entire width of the fabric substantially simultaneously. Because conventional woven fabrics for coated abrasives mostly contain at least twice this many warp yarns, no simple adaptation of the woven fabric designs to the requirements of stitch bonding machines was feasible.
It should be noted that it is possible to feed more than one warp yarn through each of the machine openings for such yarns provided in many of these machines. However, any such multiplicity of yarns fed through one opening will be bonded by the machine as if it were a single yarn. Thus the practical effect achieved by a multiplicity of yarns fed through one opening is essentially the same as that from using one plied yarn with a number of plies equal to the multiplicity of single unplied yarns. In both woven and stitch bonded fabrics, the results achieved from use of such plied yarns are not generally as satisfactory for fabric cover and for the desirable combination of strength with flexibility as can be achieved with evenly spaced finer yarns which give the same total warp tensile strength.
As noted below, the preferred machines for the fabrics of the present invention are those of the Malimo type. A publication by the manufacturer of Malimo machines, "Sewing-Knitting Machines MALIMO Technical Possibilities and Technology" describes the general range of operating conditions possible for machines of this particular type. A copy of this publication is attached and is hereby incorporated herein by reference. As may be seen from FIG. 3 in Part III, Section 3.1 of this publication, the warp and fill yarns laid out by the machine are straight and not interlaced with each other. The description of mechanical characteristics of Malimo machines given immediately below condenses from this publication those characteristics believed by the applicant to be most relevant to design of fabrics suitable for use in coated abrasives. In this condensation, the term "weft" has been changed to "fill" in accordance with common United States practice, and the term "hook needle" has been shortened to "hook"; all other terms describing the mechanical parts of the machines have been taken directly from the referenced publication.
Malimo machines have three principal mechanical characteristics which limit the variety of fabric constructions available from them. The first of these limits is provided by a group of several matched mechanical structures which fix a maximum "gauge" or number of yarns per 25 mm of width for the warp yarn and stitching yarn assemblies which can be used with the machine. Twelve possible gauges from 3 to 22 are available from the manufacturer.
The second of the principal mechanical limitations of the Malimo machine is its stitch length. This can be adjusted in 20 steps within a range of 0.7 to 5 mm. It should be noted that this nominal "stitch length" is actually the projected length in the direction of the warp yarns. When a tricot style stitch is used, as was the case for the fabrics to be described here, the actual spatial orientation of the stitch is at a substantial angle to the warp yarns, and the actual length is correspondingly longer than the nominal length. In addition, because the stitch yarns form loops, the length of yarn consumed for each stitch is generally considerably longer than either the nominal or actual length. With the fabrics described below, stitch yarn length consumption was about four times warp yarn length consumption.
The third of the principal mechanical limitations of the machine is provided by the assemblies of hooks which hold the fill yarns in tension until they can be stitched to the warp. Hook units are available in linear densities from 8 to 48 hooks per 25 mm. Under the normal conditions of use as contemplated by the instructions furnished by the manufacturer, no more than one bend of fill yarns around each hook is accommodated during fabric assembly operations.
It should be noted that it is an inherent characteristic of Malimo machines to lay fill yarns in two distinct groups at symmetric small angles on opposite sides of an imaginary line perpendicular to the warp yarn array. All fill yarn counts in this description are to be understood as including both of these fill yarn groups in the count.
The above referenced and incorporated Malimo publication gives some specifics of the construction of several fabrics suitable for other uses than coated abrasives. This is the largest such description of specific stitch-bonded fabrics known to applicant.